pe Collection Efficiency in pe Collection Efficiency in CF4 RevisitedCF4 RevisitedHBD Meeting 04/15/08B.Azmoun, A.CaccavanoBNL

Set-UpSet-Up

VUV Beam from Spectrometer

Beam Splitter Mesh-CsI plane PC

Monitor PMTVessel: Vac or Gas

• Collect pe’s in “parallel plate” config—drift pe’ from PC to mesh—not into GEM holes •PMTMonitor allows a correction for changes in lamp intensity• Collection Efficiency is defined as the ratio of PC current at a given field to the max (plateau current) in Vac• The plateau current is reached quickly in Vac

Coll. Eff.Vs WavelengthColl. Eff.Vs Wavelength

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40E/N (Td)

f

6.8

9.8

9.28.8

8.2

7.6

7.2

E ph

(eV) Ne Ar

CF4

a)

Monte Carlo Simulation

J.Escada et al

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10E/N (Td)

f

Xe

ArNe

CF4

Collection efficiency f in CF4 for photoelectrons emitted from a CsI photocathode irradiated with a continuous VUV Hg(Ar) lamp with a spectrum peaked at 6.7 eV (185 nm).

A. Breskin, et alL. Coelho, et al

J.Escada et al

Real Data

• For the parallel plate config. Illustrated, in CF4, the Coll. Eff. doesn’t reach 100% even at relatively high field and the plateau is not perfectly flat• The Coll. Eff. exhibits a wavelength dependence: smaller wavelengths suffer greater losses at a given field not good for HBD • Data and MC simulation agree relatively well• BNL and Univ. of Coimbra data agree reasonably well

QE in CF4QE in CF4• Loss in pe Coll. Eff. Is reflected in lower QE in CF4 gas• Gas Transmittance was measured simultaneously during QE measurement and was nearly 100%• A wavelength scan w/ dV(DG)=300V shows the plateau value of the coll. Eff. across a wide range in wavelength

ReproducibilityReproducibilityAbsolute QE in CF4 of CsI Photocathodes

(Corrected for M esh Transparency)

0

10

20

30

40

50

60

70

80

90

100

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

Wavelength [Angstroms]

Abs

olut

e Q

E [%

]

WIS GEM PC [CF4]

1st Chicklet, [CF4], 0.78kV/cm

1st Chicklet, [CF4], 3.25kV/cm

2nd Chicklet, [CF4], 3.25kV/cm

Comparison of Gas Transmittance and Gas/Vac Current Ratio of CsI Photocathodes

0

20

40

60

80

100

120

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstroms]

I(gas

)/I(V

ac) [

%]

Upper PMT (Cross: Vac/Vac)GEM CsI PC (CF4/Vac)Upper PMT (Cross:Vac/Vac) 3 days laterGEM CsI PC (CF4/Vac) 3 days later

CsI sample photocathode Photoelectron Collection Efficiency Vs Collection Field @ 1608 Angstroms

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5

Collection Field [kV/cm]

Pho

toel

ectr

on C

olle

ctio

n E

ffici

ncy

w.r.

t. P

E c

oll.

in V

acuu

m [%

]

Plane PC [Vacuum]Plane PC [Gas: CF4] Plane PC [Gas: Ar]WIS Lg. GEM PC [Vac]WIS Lg. GEM [CF4]

• Most of the results presented were performed several times in the past and show very reproducible quantities

Integrated pe yieldIntegrated pe yield

• CsI QE folded into Cherenkov yield calc (also corrected for mesh and GEM trans.)• Vac36.6pe’s (110-200nm)• CF427.2pe’s (110-200nm)—in this case used simple linear fit to QE data

27.2 pe

Applicable to HBD?Applicable to HBD?

D-1: Calculations performed by the simulation software, MAGBOLTZ

Reference: S. Biagi, Nucl. Instr. and Meth. A421 (1999) 234

0.01

0.1

1

10

100

0.01 0.1 1 10 100Electron energy e (eV)

s (

10-1

6 cm

2 )

Ar - - - - CF4 ——

sm

sexc

sion

sion

sn 4sn 3

sm

sa

sn ind

sd

Fig. 1. Electron scattering cross-sections in Ar, Ne and CF4 used in the Monte Carlo simulation: elastic momentum transfer (σm), vibrational excitation (σν4, σν3, σνind ), electron attachment (σa), dissociation (σd), excitation (σexc), and ionization (σion).

J.Escada et al

~0.15mm1.5mm

•If pe losses are similar over distances which differ in scale by an order magnitude, then all losses must happen very early in pe’s trip through gas—need detailed simulation which takes into account all interaction cross sections as pe traverses gas